Phased array antennas consist of an array of fixed radiating elements in which the radiation pattern of a microwave beam is determined by the phase relationship of the signals that excite the radiating elements. The radiating elements include phase shifters operating under computer control so that the beam is scanned in azimuth and elevation without mechanical movement of the radiating elements. The phase shifting elements are preferably ferrites which are a class of materials consisting of compressed and sintered powders of a magnetic material, chiefly ferric oxide, and one or more metals. The ferrite materials may come in various combinations, but three major classes of such ferrites are spinels, garnets, and hexagonal ferrites depending on the crystal structure of the ferrite.
Of these various classes of ferrites, the garnet ferrites have become very useful and very popular because they have the most advantageous characteristics --namely, low RF losses, and they are smaller, lighter, and more reliable than other types of ferrite phase shifters. The ferrite elements are positioned in a wave guide and control the phase of the incoming RF signal by means of a magnetic field periodically applied along the ferrite element. The magnetic fields may be developed, in the case of metallized cylindrical ferrite rods, by surrounding the ferrite rod with a coil or by passing a latch wire through the center of a rectangular ferrite annulus. DC latching pulses are applied to the latch wire to produce the magnetic field which shifts the phase of the incoming RF signal.
While garnet ferrites are useful in phased array antenna systems, demand for improved performance requires that the phase shifters handle ever increasing power levels. Ultimately, the power handling capacity is limited by the ability to maintain the operational temperature of the phase shifters within prescribed limits. That is, since heat is generated in the garnet ferrite cores, the cores are subject to mechanical stresses due to thermally induced bending. These stresses reduce the performance of the phase shifter. The problem of thermally induced stresses are exacerbated as the power level for these garnet ferrite cores has reached 25 watts and higher.
Various cooling schemes have been developed in the past to remove the heat from the core in an attempt to limit the thermally induced stresses. Among these schemes are forced-air cooling of the phase shifter housing and positioning the garnet core along a surface of the housing so that the housing acts as a heat sink conducting heat to the exterior where the forced air cooling removes the heat. However, as the power levels increase, convective cooling becomes very inefficient. Furthermore, as the density of the radiating elements increases in higher frequency radar applications, the air path between the phase shifter housing virtually disappears making forced air cooling very difficult.
For example, single surface conductive cooling of the core by placing one surface against the housing is adequate up to approximately 25 watt average dissipated power for 10 inch long S band garnet cores. Above this power level, the thermally induced bending stress resulting from the nonsymmetric temperature gradient in the core because only one surface is heat sinked to the wall degrades phase shifter performance. Furthermore, the reliability of the ferrite phase shifters is also drastically reduced since the bending and other stresses can result in micro-cracks and catastrophic failure of the core.
Enlarging the ferrite core and the housing to reduce heat density in the core and hence, the thermal stresses, is not a very palatable solution as this results in a large and heavy antenna design. A need therefore exists for a small, compact ferrite phase shifter design which is cooled in such a manner that it is capable of handling very high power levels, in excess of 25 watts, without subjecting the ferrite core to the undesired thermal bending stresses due to uneven heat gradients across the cross-section and length of the core. Applicant has found that all of these desirable characteristics may be realized by mounting the ferrite phase shifter in a hermetically sealed housing and immersing it in a liquid to provide symmetrical heat transfer from all sides of the ferrite phase shifter to the walls of the housing. In this fashion, the ferrite phase shifter is cooled symmetrically and is not subject to any thermal stresses.